.beta.-amyloid peptide (A.beta.) is the major protein component of senile plaques and cerebrovascular amyloid deposits from Alzheimer disease (AD) patients (Glenner, G. G., and Wong, C. W., Biochem. Biophys. Res. Comm. 120:885-890, 1984; Wong, C. W., et al., Proc. Natl. Acad. Sci. USA. 82:8729-87322, 1985). Deposition of A.beta. in the form of amyloid fibrils is believed by many to be causally linked to the disease (Joachim, C. L., and Selkoe, D. J., Alzheimer Dis. Assoc. Disord. 6:7-34, 1992). A.beta. is toxic to cultured neuronal cells, and this toxicity has been linked to the aggregational and/or conformational status of the peptide (Pike, D. J., et al., J. Neurosci. 13:1676-1687, 1993; Simmons, L. K., et al., Mol. Pharmacol. 45:373-379, 1994; Ueda, K., et al., Brain Res. 639:240-244, 1994).
Under physiological conditions, A.beta. readily aggregates into fibrils with a cross-.beta. sheet conformation. Coincident with the conversion of monomeric A.beta. to fibrillar A.beta. is a transition from random coil to .beta.-sheet (Terzi, E., et al., J. Mol. Biol. 633-642, 1995). Several features of A.beta. affect the facility of this transition. The peptide is amphophilic, with a hydrophilic N-terminus and hydrophobic C-terminus; the length of the latter affects the rate of aggregate formation (Jarrett, J. T., et al., Biochemistry 32:4693-4697, 1993). In addition, a short hydrophobic stretch at residues 17-21 appears to be critical in the formation of fibrillar structure (Hilbich, C., et al., J. Mol. Biol. 228:460-473, 1992; Fraser, P. E., et al., Biophys. J. 60:1190-1201, 1991), with charged residues adjacent to this region also contributing to fibril formation (Hilbich, C., et al., supra, 1992; Fraser, P. E., et al., J. Mol. Biol. 244:64-73, 1994).
Aggregation likely proceeds via formation of a "nucleus" to initiate fibril formation followed by fibril elongation (Jarrett, J. T., et al., supra, 1993; Shen, C.-L., and Murphy, R. M. Biophys. J. 69:640-651, 1995; Lomakin, A., et al., Proc. Natl. Acad. Sci. USA 93:1125-1129, 1996).
One strategy for developing lead candidates for drugs to treat AD patients is to screen for small-molecule compounds that disrupt A.beta. aggregation and thereby, presumably, interfere with its toxicity. Sulfonated dyes such as Congo red and related sulfonate anions reportedly disrupt A.beta. aggregation and reduce A.beta. toxicity (Pollack, S. J., et al., Neurosci. Lett. 197:211-214, 1995; Kisilevsky, R., et al., Nature Medicine 1:143-148, 1995). The cationic surfactant hexadecyl-N-methylpiperidinium bromide inhibits A.beta. fibril formation, possibly by binding to a site on A.beta. necessary for A.beta. self-assembly (Wood, S. J., et al., J. Biol. Chem. 271:4086-4092, 1996). .beta.-Cyclodextrin, which has an affinity for hydrophobic groups, partially reduces A.beta. toxicity (Camilleri, P., et al., FEBS Letts 341:256-258, 1994).
An alternative approach is to use a fragment of A.beta. to disrupt A.beta. fibril formation. Recently, it was reported that a pentapeptide KLVFF, containing the 16-20 sequence of full-length A.beta., binds to and disrupts fibril formation (Tjernberg, L. O., et al., J. Biol. Chem. 271(15):8545-8548, 1996). An octapeptide, QKLVTTAE, with substitutions for the two Phe residues at positions 19 and 20, inhibited fibril formation at a 10-fold molar excess, a result that was attributed to weak interactions between the octapeptide and monomeric A.beta. (Hughes, S. R., et al., Proc. Natl. Acad. Sci. USA 93:2065-2070, 1996). In both cases, fibril inhibition was assessed by electron microscopy. Effects of the peptide fragments on A.beta. toxicity were not reported.
Needed in the art is an improved inhibitor of .beta.-amyloid toxicity.